The present invention relates to the field of amplifiers, and in particular, to a compact turn-around stage with Class AB behavior, low power consumption, low noise and improved offset performance.
An integrated amplifier circuit such as an opamp is usually constructed from a small chip of semiconductor material upon which an array of active/passive components have been constructed and connected together to form a functioning circuit. An integrated amplifier circuit is generally encapsulated in a plastic housing (chip) with signal, power supply, and control pins accessible for connection to external electronic circuitry. Typically, input signals transmitted to the integrated amplifier circuit via selected input pins are processed by active and passive components in different stages, e.g., input and turn-around, and the processed signals are then applied to selected output pins using an output stage.
The enormous growth of high-speed communication and high data rate image processing applications, requiring high-speed, low power and small size, has created a demand for miniaturized high-speed amplifiers that can operate at low voltages. To maximize the dynamic range at low supply voltages for this use, it is desirable that the output voltage range for this type of amplifier should be as large as possible. Preferably, the output voltage range of the amplifier would extend from one rail to the other rail of the power supply.
Class-AB circuitry is used in amplifiers that employ both bipolar and/or MOS components. A Class AB circuit can deliver to and pull from a load a current that is larger than the DC quiescent current flowing into the circuit. Class AB circuitry is preferred in output stage of a low-power high speed amplifier because it improves power efficiency by maximizing the output drive current with a relatively low quiescent current. For example, the drive current for a Class AB circuit might be 100 milliamps and the quiescent current could be 1 milliamps. Also, Class AB circuitry exhibits good linearity over the entire output voltage range.
A turn-around or level-shift stage is an often neglected, but crucial part of an amplifier. Its main purpose is providing a level shift function, such that the input common mode voltage range of the amplifier is independent of the output voltage. Because the turnaround stage often directly follows an input stage, the supply current for the turn-around stage often needs to be high enough to compensate for the total current swing of the input stage. In addition, the turn-around stage is usually an important contributor to input non-ideal parameters such as offset and noise. In many cases, the turnaround stage can be the dominant factor for these parameters, and sometimes even more so than the input stage itself.
In accordance with the invention, an apparatus for an operational amplifier with reduced quiescent current requirements, and therefore reduced offset and noise contributions, is provided. An input stage is adapted for receiving an input signal.
Briefly described, a turn-around stage is provided that accepts the full current swing from an input stage while maintaining a low quiescent current. The circuitry provides Class AB operation with quiescent currents that are significantly less than the maximum signal current so that overall power consumption is significantly reduced. Also, the amount of noise and offset contributions of the circuit are reduced by reducing the transconductances associated with transistors included in the turn-around stage.
In addition, the transconductance associated with various transistors included in the turn-around stage are reduced by reducing the power consumption of the turn-around stage. High transconductance values contribute to high offset and noise values for an amplifier. By reducing the power consumption of the turn-around stage, the adverse effects of offset and noise in the amplifier are minimized.
Also, the turn-around stage described herein is configured to operate at a lower operating current than the input stage, while retaining the ability to accept large signal currents from the input stage. Thus, the quiescent current supplied to the turn-around stage is not required to be commensurate with the signal currents from the input stage.
Additionally, the open loop gain for the amplifier is increased by reducing the quiescent current that is consumed by the turn-around stage. In particular, when the quiescent currents of certain transistors are reduced, their output impedance increases, and results in increased open loop gain.
The invention may also be implemented as methods that perform substantially the same functionality as the embodiments of the invention discussed above and below.
These and other features as well as advantages, which characterize the invention, will be apparent from a reading of the following detailed description and a review of the associated drawings.